Long-term real-time chemical characterization of submicron aerosols at Montsec (southern Pyrenees, 1570 m a.s.l.)

摘要

Real-time measurements of inorganic (sulfate, nitrate,ammonium, chloride and black carbon (BC)) and organic submicron aerosols(particles with an aerodynamic diameter of less than 1 μm) from acontinental background site (Montsec, MSC, 1570 m a.s.l.) in the westernMediterranean Basin (WMB) were conducted for 10 months (July 2011–April2012). An aerosol chemical speciation monitor (ACSM) was co-located withother online and offline PM measurements. Analyses of the hourly,diurnal, and seasonal variations are presented here, for the first time, forthis region.Seasonal trends in PM components are attributed to variations inevolution of the planetary boundary layer (PBL) height, air mass origin, andmeteorological conditions. In summer, the higher temperature and solarradiation increases convection, enhancing the growth of the PBL and thetransport of anthropogenic pollutants towards high altitude sites.Furthermore, the regional recirculation of air masses over the WMB creates acontinuous increase in the background concentrations of PM componentsand causes the formation of reservoir layers at relatively high altitudes.The combination of all these atmospheric processes results in a highvariability of PM components, with poorly defined daily patterns,except for the organic aerosols (OA). OA was mostly composed (up to 90%)of oxygenated organic aerosol (OOA), split in two types: semivolatile(SV-OOA) and low-volatility (LV-OOA), the rest being hydrocarbon-like OA(HOA). The marked diurnal cycles of OA components regardless of the air massorigin indicates that they are not only associated with anthropogenic andlong-range-transported secondary OA (SOA) but also with recently producedbiogenic SOA.Very different conditions drive the aerosol phenomenology in winter at MSC.The thermal inversions and the lower vertical development of the PBL leaveMSC in the free troposphere most of the day, being affected by PBL airmasses only after midday, when the mountain breezes transport emissions fromthe adjacent valleys and plains to the top of the mountain. This results inclear diurnal patterns of both organic and inorganic concentrations. OA wasalso mainly composed (71%) of OOA, with contributions from HOA (5%)and biomass burning OA (BBOA; 24%). Moreover, in winter sporadiclong-range transport from mainland Europe is observed.The results obtained in the present study highlight the importance of SOAformation processes at a remote site such as MSC, especially in summer.Additional research is needed to characterize the sources and processes ofSOA formation at remote sites.